1. Field of the Invention
The present invention relates to methods of manufacturing lenses using pressing molds.
2. Description of the Prior Art
Recently, attempts have been made to mold lenses having an optical mirror surface without grinding and polishing. These attempts have used a mold made of a specific material and formed to have an optical mirror surface. The molding has been performed in a non-oxidizing atmosphere. As the material for the mold, for example, the use of glassy carbon is disclosed in U.S. Pat. No. 3,833,347, the use of SiC or Si.sub.3 N.sub.4 is disclosed in U.S. Pat. No. 4,139,677, and the use of a mixture of SiC and carbon is disclosed in U.S. Pat. No. 4,168,961.
In the molding method disclosed in U.S. Pat. No. 3,833,347, or the like, a block of glass is inserted into a mold. Then the inside of a chamber surrounding the mold is evacuated and non-oxidizing gasses are introduced into the chamber. The temperature of the mold is then raised to a value in the vicinity of the glass softening point (that is, a temperature corresponding to a glass viscosity of 10.sup.7.65 poise) and is maintained for 1 to 5 minutes. The glass is then molded by applying a force to the mold. The temperature of the mold is then reduced to a value lower than the glass transition temperature while the force on the mold is maintained so that the molded glass does not deform. Then, the force is removed and the mold is cooled to about 300.degree. C. in order to prevent oxidation of the mold. Finally, the mold is opened and the molded lens is taken out. A problem exists, however, in that the steps of this process must be performed sequentially, resulting in a long cycle time.
Japanese Patent Application Laid-Open No. 8134/1983 discloses a method of molding glass products in which a wide range of mold materials can be employed and the time taken for a pressing cycle is shorter. In this method, a glass preform having a shape approximate to that of the final product is used and after the glass preform and a mold are heated to a temperature producing a glass viscosity not smaller than 10.sup.8 poise but not larger than 10.sup.12 poise, a force is applied to mold the glass preform. The SHAPED GLASS is then REMOVED from the mold at a temperature where glass exhibits a viscosity of less than 10.sup.13 poise, and then the shaped glass is annealed.
However, in both of these methods of molding glass products, cooling is performed with a force applied to the mold. Accordingly, the time required for the pressing step has been long and overall manufacturing speed has improved little.
When it is desired to obtain a high surface accuracy, it is important to prevent shrinkage. Japanese Patent Publication No. 378/1981 teaches pressing molten glass with a mold maintained at a predetermined temperature between the softening point and the transition point for 20 seconds or more to prevent shrinkage. However, this method does not completely prevent shrinkage from occurring because the glass is taken out without reducing the pressure after pressure at a relatively early stage.